FIG. 1 illustrates a prior art half-bridge LLC resonant converter 100. The resonant converter 100 comprises a square wave signal generator 101, a resonant network 102, an isolated transformer T and a rectifier network 103. The square wave signal generator 101 is built as a half-bridge type and comprises a high-side switch M1 and a low-side switch M2. The square wave signal generator 101 is configured to convert an input DC voltage VIN into a square wave signal Vd by driving the high-side switch M1 and the low-side switch M2. The resonant network 102 receives the square wave signal Vd and is coupled to the load through the rectifier network 103 to provide an output DC voltage Vo.
In prior art, the resonant converter with primary side control is configured to detect whether a capacitive mode or an inductor mode based on phase difference between a primary input current Ip flowing into the resonant network 102 and the square wave signal Vd applied in the resonant network 102. FIGS. 2A and 2B illustrate schematic waveform diagrams of the resonant converter 100 in the inductor mode and capacitive mode, respectively. As shown in FIG. 2A, the primary input current Ip lags the square wave signal Vd, the resonant converter 100 works in the inductive mode, the high-side switch M1 can be turned ON at zero voltage. As shown in FIG. 2B, the primary input current Ip leads the square wave signal Vd, the resonant converter 100 works in the capacitive mode, the body diode of the high-side switch M1 presents reverse-recovery because of the hard switching, that causes high power dissipation. Furthermore, the slow reverse recovery may allow severe shoot-through of the high-side switch M1 and the low-side switch M2, resulting in high current spikes and causing the switches to fail. However, the prior mode detection method is hard to apply in the resonant converter with a secondary side control.